Recent development in mobile communications technologies has been towards increased bandwidths and faster data rates. The GSM (Global System for Mobile Communications) has been one of the most successful communications technologies ever. However, as the relatively slow transmission speed of GSM has been a bottleneck for creating better services to the consumer market, a lot of effort has been put into developing new faster technologies for mobile communications. One such example is EDGE (Enhanced Data rates GSM Evolution). The standardization of EDGE was completed at the European Telecommunications Standards Institute in 1997.
For GSM/EDGE Radio Access Network (GERAN), there have been several new challenges to overcome. Higher data rates are achieved in part by changing channel coding. The transmission in a TDMA (Time Division Multiple Access) system takes place in time frames. Each frame can be shared among users by dividing the frame into time slots. A TDMA time frame thus comprises physical channels used to physically transfer information from one place to another. The contents of the physical channels form logical channels, which can be divided into traffic and control channels. The control channels can be further divided into dedicated and common channels. The dedicated channels are used for traffic and signaling between the network and the Mobile Stations (MS), whereas the common channels are used for broadcasting different information to the MS and for setting up signaling channels between the Mobile Switching Center/Visitor Location Register (MSC/VLR) and the MSs. Over the radio path, different types of signaling channels are used to facilitate the discussions between the MSs and the Base Transceiver Stations (BTSs), Base Station Controllers (BSCs), and the MSC/VLR. The logical channels are mapped onto physical channels as described in the technical specification 3GPP TS 45.002 (GERAN Multiplexing and multiple access on the radio path).
In the GSM system, the modulation method used is a phase modulation known as Gaussian Minimum Shift Keying (GMSK). In GMSK, the phase of a true bit is shifted 90°, whereas the phase of a false bit is not shifted. With the increasing data rates of EDGE, new 8 Phase Shift Keying (8-PSK) with 8 possible shift values has been introduced (3GPP TS 45.004). Each of the shift values corresponds to a certain symbol consisting of 3 bits.
The two types of speech traffic channels used in the GSM are the Full Rate GMSK Traffic Channel (TCH/F) and the Half Rate GMSK Traffic Channel (TCH/H). For the TCH/F channels, the voice codecs normally used are the Full Rate (FR) and the Enhanced Full Rate (EFR) codecs. The EFR speech coder provides the best quality of voice. For the TCH/H channels a Half Rate (HR) coder is normally used, which consumes less bandwidth as compared to the FR codecs. The HR coder can therefore be used to serve a double number of subscribers on a half rate speech traffic channel as compared to an FR coder on a full-rate speech traffic channel.
In order to achieve a better voice quality, a new Adaptive Multi-Rate (AMR) coder has been introduced (Release 1998). Further, the introduction of AMR on TCH/H channels utilizing 8-PSK (O-TCH/H channels) has been considered. However, there has not been any channel coding for the AMR signaling frames defined for such traffic channels (O-TCH/H). The AMR signaling frames are listed in Table 1.
TABLE 1The different AMR signaling frames used on half rate channels.AMR signalingframePurposeSID_FIRST_P1indicates end of speech, start of DTX (1st part)SID_FIRST_P2indicates end of speech, start of DTX (2nd part)SID_FIRST_INHinhibits the second part of aSID_FIRST_P1 if aspeech onset occursONSETtells the codec the mode ofthe first speech frameafter DTXSID_UPDATEconveys comfort noise parameters during DTXSID_UPDATE_INHinhibits the second part of aSID_UPDATE frame ifa speech onset occursRATSCCH_MARKERidentifies RATSSCH framesRATSCCH_DATAconveys the actual RATSSCH message
It is not possible to use GMSK for the AMR signaling frames and 8-PSK for the traffic at the same time, because some of the signaling frames, such as for the ONSET signaling messages, share the same bursts as the speech.
As the same AMR signaling frames are needed for the new half rate channels using 8-PSK modulation (O-TCH/H), a new channel coding for these frames has to be introduced. A straightforward solution to this problem is now discussed with reference to FIG. 1. The numbers under the data flow arrows in the figure denote the number of bits included in a bit block used in the system. The reader is kindly referred to the document 3GPP TS 45.003 V5.1.0 (GSM/EDGE Radio Access Network; Channel Coding) and to the references therein about the different messages referenced below.
A crucial part of the system is the Channel Coder 100 in FIG. 1A. Usually a block to be transmitted includes Inband Data 101 consisting of two bits. These bits are coded in a coding block 102 using predefined code words, which must be 48 bits in length in order to correspond to the reserved block length. Occasionally, the bits to be transmitted further include Identification Marker Sequences 103 consisting of 9- or 11-bit sequence. Eleven bits are used for the RATSSCH_MARKER, for which a repetition of 58 times in the repetition block 104 is required to get the correct total block length of 636 bits. For the other AMR signaling frames a repetition of 71 times is required in the repetition block 104. For the AMR signaling frames SID_UPDATE, which convey Comfort Noise parameters during a Discontinuous Transmission (DTX) period, and for the RATSCCH_DATA AMR signaling frame the Comfort Noise parameters 105 also need to be coded. A Cyclic Redundancy Check (CRC) is performed in the check block 106 to protect the Comfort Noise against transmission errors. This checksum (14 bits) is added to the Comfort Noise parameters (49 bits total), and the result is fed through a convolutional encoder block 107, which increases the block length to 636 bits.
All signals coming from the Channel Coder 100 are multiplexed in a multiplexing block 108. The total number of bits to be sent in a block is either 684 bits or 1368, depending on the AMR signaling frame. The AMR signaling frames are mapped in the mapping block 109 to 8-PSK symbols, which modifies the block size to 228 or 456 symbols. The symbols resulting from the signaling frames are then interleaved in the interleaving block 110 together with blocks from other frames, which may be speech frames, for example. After the interleaving a burst will be formatted in the burst formation block 111. Then the burst is modulated in a modulation block 112 and directed to the transmission block 113.
In FIG. 1B, after receiving a signal in a receiving block 129, the signal must be demodulated in the demodulation block 130. The content of the original burst has to be recovered in the recovery block 131. Because the burst consists of interleaved symbols, they must first be fed through the deinterleaving block 132 and then converted back to bits in the converting block 133. Before the messages can be passed to the Channel Decoder 120, the signaling must be de-multiplexed in the de-multiplexing block 134 so that the Inband Data part 136 is decoded in the codeword decoder block 135, and the Identification Marker Sequence 138 in the Identification Marker decoder block 137. If the AMR signaling frame includes Comfort Noise parameters, they are then decoded in the corresponding decoding block 139, and the CRC bits are verified in the verification block 140. Only after this are the Comfort Noise parameters 141 obtained.
The drawback of the solution described above is that both high rate convolutional codes and high rate block codes are required. The convolutional coder 107 encodes the 49 bits sequence into 636 bits, and the block coder 102 encodes the 2 bits into 48 bits. In the reverse direction, the convolutional decoder 139 decodes the 636 bits into the 49 bits, and the code word decoder 135 decodes the 48 bits into the 2 bits. These so-called fast-rate conversions are not desirable, as they increase the development cost and demand larger coding tables in the network elements and terminals. They are also computationally heavy and consume a lot of memory.
The rate of the convolutional encoder 107 for the Comfort Noise is 1/12, which is more demanding than the ¼ for the GMSK. The constraint length can also be increased from k=5 to k=7, as is done for speech, and the existing polynomials G4-G7 can be used. The interested reader may find descriptions of the polynomials and the constraint lengths in the document 3GPP TS 45.003 V5.1.0 (GERAN Channel Coding). As explained above, the shorter 9 bit sequence of the identification marker has to be repeated 71 times.
The purpose of the present invention is to address the problem discussed above. This can be achieved using a method and system for processing AMR signaling frames as described in the independent claims.